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24 May 2021 Edvard@Fysik.Su.Se Orsodn Uhr Dadm¨Ortsell Edvard Author: Corresponding Tension Draft version May 26, 2021 Typeset using LATEX twocolumn style in AASTeX631 The Hubble Tension Bites the Dust: Sensitivity of the Hubble Constant Determination to Cepheid Color Calibration Edvard Mortsell,¨ 1 Ariel Goobar,1 Joel Johansson,1 and Suhail Dhawan2 1Oskar Klein Centre, Department of Physics, Stockholm University Albanova University Center 106 91 Stockholm, Sweden 2Institute of Astronomy University of Cambridge Madingley Road Cambridge CB3 0HA United Kingdom ABSTRACT Motivated by the large observed diversity in the properties of extra-galactic extinction by dust, we re- analyse the Cepheid calibration used to infer the local value of the Hubble constant, H0, from Type Ia supernovae. Unlike the SH0ES team, we do not enforce a universal color-luminosity relation to correct the near-IR Cepheid magnitudes. Instead, we focus on a data driven method, where the measured colors of the Cepheids are used to derive a color-luminosity relation for each galaxy individually. We present two different analyses, one based on Wesenheit magnitudes, a common practice in the field that attempts to combine corrections from both extinction and variations in intrinsic colors, resulting in H0 = 66.9 ± 2.5 km/s/Mpc, in agreement with the Planck value. In the second approach, we calibrate using color excesses with respect to derived average intrinsic colors, yielding H0 = 71.8 ± 1.6 km/s/Mpc, a 2.7 σ tension with the value inferred from the cosmic microwave background. Hence, we argue that systematic uncertainties related to the choice of Cepheid color-luminosity cali- bration method currently inhibits us from measuring H0 to the precision required to claim a substantial tension with Planck data. Keywords: Cepheid distance (217), Hubble constant (758), Type Ia supernovae (1728), Interstellar dust extinction (837) 1. INTRODUCTION The tension between other measurements is not as sig- As is well known, there is a tension between the value nificant: Calibrating the absolute SNIa magnitude us- of the Hubble constant as inferred from small and large ing the tip of the red giant branch (TRGB) observations distance measurements, most significantly between the gives H0 = 69.6 ± 1.6 (Freedman et al. 2019), right be- values inferred from Type Ia supernova (SNIa) distances tween the Cepheid calibrated SNIa and the CMB in- arXiv:2105.11461v1 [astro-ph.CO] 24 May 2021 to redshifts z ∼ 0.1 calibrated by Cepheid observations, ferred values. as measured by the SH0ES team, and the distance to Another local estimate of the expansion rate of the the cosmic microwave background (CMB) decoupling Universe is derived from the amplitude of the gravita- surface at z ∼ 1090, as measured by the Planck satel- tional wave signal GW170817, the merger of a binary neutron-star system located to the galaxy NGC 4993 lite. The former yields H0 = 73.2 ± 1.3 (in units of km/s/Mpc used from now on) (Riess et al. 2021) and at z = 0.010 through the electromagnetic counterpart, AT2017gfo, yielding a Hubble constant with relatively the latter H0 = 67.4±0.5 (Aghanim et al. 2020); a4.1 σ +12.0 tension. large uncertainties of H0 = 70.0−8.0 (Abbott et al. 2017). A different route involves gravitational lensing. Time Corresponding author: Edvard M¨ortsell delays in the TDCOSMO sample of seven lensed quasars [email protected] +5.6 yield H0 = 74.5−6.1 (Birrer et al. 2020). Constraining 2 Mortsell¨ et al. the galaxy lens mass profiles, using kinematics obser- present before CMB photon decoupling, e.g., new ther- vations of an independent set of gravitational lenses in mal relativistic species or early dark energy, or by reduc- the Sloan Lens ACS sample (SLACS), lowers the value ing the sound speed. However, such modifications are +4.1 to H0 = 67.4−3.2, assuming that the TDCOSMO and severely constrained when taking the full CMB power SLACS galaxies are drawn from the same parent pop- spectrum into account. Attempts to shift the CMB ulation. These results are illustrated in Figure 1, from value also involve changing the expansion history at red- which is evident that only the Cepheid calibrated SNIa shifts z < 1090, with modest success since the expansion distance scale from SH0ES is in definite tension with the rate is tightly constrained by SNIa and baryonic acoustic CMB inferred distance. oscillation (BAO) observations. Given that the proposals mentioned above require substantial modifications of the current concordance cosmological model, and still fail in relieving the full 1000 CMB tension, we investigate the Cepheid-SNIa value and its 100 Lensing uncertainties, see also Follin & Knox (2018); Efstathiou SH0ES (2020). In particular, we concentrate on dust extinction, TRGB-SNIa 10 GW affecting all astronomical observations in the optical and near infrared (NIR) regime. We focus on a a very spe- z 1 cific assumption made by the SH0ES team throughout their series of publications, namely that there is a uni- 0.1 versal reddening law in all galaxies. 0.01 2. REVISITING DUST EXTINCTION CORRECTIONS 60 65 70 75 80 85 H In spite of the critical importance for precision cosmol- 0 ogy, the current understanding of light attenuation in the interstellar medium (ISM) of galaxies remains very Figure 1. The Hubble constant as inferred from distances measured to the CMB, strongly lensed quasars, SNIa cal- limited. In comparison, the Milky Way (MW) ISM has ibrated with Cepheids (SH0ES) and the TRGB, and the been studied in great detail, including the properties of gravitational wave signal GW170817. The redshift corre- dust grains responsible for dimming of light(see Draine sponds to the mean redshift of the distances employed in the 2003, for a review). In particular, several MW redden- method, slightly shifted where needed to avoid overlap. The ing laws have been devised, among these Cardelli et al. major tension is between the values inferred from Cepheid (1989) (CCM), O’Donnell (1994) and Fitzpatrick (1999) calibrated SNIa by the SH0ES team and CMB observations. (F99). They have in common the use of a single param- BV eter, the total to selective extinction coefficient, RV , as a proxy for the grain composition and size distri- The value inferred from the CMB depends on the en- bution, where the attenuation in the optical V-band tire expansion history of the Universe, whereas the SNIa relates to the color excess E(B − V) ≡ AB − AV as BV measurement only depends on the local expansion rate AV = RV E(B − V), here referred to as the ”CCM- BV it sets out to measure. On the other hand, the infer- relationship”. Low values of RV indicate a steep wave- BV ence from SNe Ia depends on a combination of a larger length dependence, while large RV correspond to gray number of astrophysical probes. Therefore, attempts to extinction. BV modify the CMB inferred H0 usually rely on modifica- While an average hRV i =3.1 for the MW is found in tions of the cosmological model, whereas the SNIa value most studies, significant variations are found in individ- BV is usually studied with emphasis on possible systematic ual lines of sight in the galaxy, ranging from RV ≈ 2 in BV effects concerning the local distance measurements. some diffuse sight lines, to RV ≈ 6 in dense molec- At least in principle, the CMB value can be increased ular clouds (Fitzpatrick 1999). The diversity in the by various departures from the concordance cosmolog- MW has been confirmed by Nataf et al. (2016), who BV ical constant and cold dark matter, ΛCDM model, see find significantly lower values of RV in the Galactic e.g. M¨ortsell & Dhawan (2018); Knox & Millea (2020). bulge. Moving the scope outside the MW, a study Options include decreasing the physical size of the sound by Gordon et al. (2003) of the extinction in the Mag- horizon used to measure the distance to z = 1090. ellanic Clouds found that a small number of LMC ex- This can be accomplished by adding sources of energy tinction curves are consistent with the CCM relation- The Hubble Tension Bites the Dust 3 ship, but the majority of the LMC and all the SMC curves are not. Fausnaugh et al. (2015) report a gray 1.0 extinction law for NGC 4258 in the line of sight of BV the Cepheids, RV ∼ 4.9, although they caution that 0.8 this could be the result of unresolved systematics. For more distant galaxies, observed SNIa colors highlight 0.6 the observed diversity in extinction properties, rang- VI H ing from RBV ∼ 1 to values consistent with the MW R V 0.4 FTZ19 average (see Krisciunas et al. 2006; Nobili & Goobar B18 SH17 2008; Goobar et al. 2014; Amanullah et al. 2014, 2015; LMC1 Burns et al. 2018, and references therein). For the 0.2 LMC2 FA15 SNe Ia in the Hubble flow, color corrections are based 0.0 on the SALT2 lightcurve fitter (Guy et al. 2007), which 1 2 3 4 5 6 again differ from the CCM parameterization, but are RBV BV V most consistent with values of RV ∼ 2.5 (see e.g., Biswas et al. 2021, and references therein), although VI BV dust extinction differences between host galaxy envi- Figure 2. RH vs RV relation and 1σ scatter from the ex- tinction law derived by Fitzpatrick et al. (2019) (FTZ19), ronments has been suggested as an explanation for BV BV extrapolated to RV > 6 and RV < 2. Such low val- a systematic ”mass step” in the derived distances ues have been inferred from SNIa colors (Burns et al. 2018, (Brout & Scolnic 2021; Johansson et al. 2021). BV B18). Also shown are derived RV values from a MW stel- To minimize the impact from extinction correction lar sample in Schlafly et al.
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